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The ecological and evolutionary significance of reproductive traits in corals

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Schmidt-Roach, S (2013) The ecological and evolutionary significance of reproductive traits in corals. PhD thesis, University of Tasmania.

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Abstract

In this thesis, I explore species boundaries in corals and I investigate how distinct reproductive
strategies may in
uence gene
ow within and among species and ultimately how this
aects processes of genetic isolation and speciation in corals.
Reconstructing genealogical relationships among colonies of pocilloporid corals sampled along
Eastern Australia in chapter 2 conrmed the hypothesis that the extensively studied species
P. damicornis constitutes a cryptic species complex rather than a single reproductively and
morphologically plastic species. Haplotype networks computed from two mitochondrial DNA
regions (CR, ORF) recovered at least ve genetically distinct mitochondrial lineages within
P. damicornis corresponding to distinct morphotypes previously considered ecomorphs. In
addition, brooding and spawning observations on the Great Barrier Reef (GBR), suggested
that the ancestral reproductive strategy in Pocillopora is broadcast spawning, while asexual
brooding seems to be an apomorphic trait, characteristic to a single genetic clade that
maintains a mixed mode of spawning and brooding. This conrmed the hypothesis for
chapter 3 that P. damicornis (or hidden species within) brood larvae in Eastern Australia
which are solely asexual. Further, the brooding clade contained three genetic lineages; two
of them occur in sympatry in the Great Barrier Reef (GBR) and maintain brooding periods
at opposite lunar phases. These GBR lineages showed clear nuclear divergences (HSP70,
ITS2) conrming reproductive isolation likely due to dierences in reproductive timing. In
contrast, other lineages within Pocillopora lacked sharp nuclear divergence despite belonging
to dierent mitochondrial clades and exhibiting dierent reproductive traits, indicating that
occasional hybridisation occurs even between distant, otherwise well-dened lineages.
Further investigations, including additional Pocillopora species from Australia, conrmed
mitochondrial molecular phylogenies are congruent with groups based on gross-morphology
and symbiont association, therefore re
ecting species-level dierentiation. Fine scale
morphological variation, particularly the shape and type of columella, added additional
support for the dierentiation of genetic lineages and provided an excellent signature of
the evolutionary relationships amongst them.
The apparent misinterpretation of taxonomical units within P. damicornis may explain the
perceived variation in the ecology, biology and life history of this species across its range. In
order to provide appropriate taxonomic units for future references, colonies genotyped from
Eastern Australia were compared to type specimens and original descriptions, which allowed
for the revision of the taxonomic status of eight closely related species to and within the P.
damicornis species complex, including the description of two new taxa. This conrmed the
hypothesis that genetic lineages identied within P. damicornis correspond to taxonomic
morphospecies (Chapter 4 & 5). Much of what we know about reproductive relationships, including the ability of species
to hybridise or self-fertilise, stems from ex situ fertilisation experiments and relies on the
assumption that laboratory trials adequately re
ect processes that are occurring in natural
environments. However, this has never been tested empirically. Given that reproductive
relationships within and among species play such an important role in the evolution of species,
it is critical that we fully understand the links between laboratory trials and real world
processes in order to make accurate predictions about species relationships and population
dynamics. In chapter 6 I assessed the nature of fertilisation ex - versus in-situ in the
hermaphroditic broadcast-spawning coral Goniastrea favulus, using seven newly developed
microsatellite markers to compare estimates of selng and outcrossing under natural and
experimental conditions. In particular, I tested for the hypotheses that self-fertilisation
represents an important reproductive strategy in Goniastrea favulus and that G. favulus
favours non-self over self-fertilisation in situations of sperm choice. In the presence of selfand
non-self sperm, the ex situ experiments showed high levels of selng in G. favulus (30%),
consistent with the known ability of this species to self-fertilise. However genotyping larvae
fertilised under natural conditions revealed low levels of selng on the reef (<4%), suggesting
that the occurrence of selng in this species may have been signicantly overestimated in
laboratory trials and that laboratory experiments may fail to re
ect real world conditions and
lead to a distorted perception of reproductive relationships within and among coral species.
In particular, the importance of inter-specic hybridisation between some coral species gained
from similar laboratory fertilisation trials may have been similarly overestimated.
Hybridisation has been thought to be an important aspect of coral reproduction and has
been considered, in part, to explain blurred morphological species boundaries in many
genera and consequently reticulate evolution has been considered central to coral speciation.
However, conclusions from ex situ trials that hybridisation occurs commonly, and hence its
signicance for coral evolution, may well be misleading. Nonetheless, my genetic data on
Pocillopora suggest that occasional introgressive hybridisation at evolutionary time scales is
likely to occur between some coral species. Within the genus Pocillopora semi-permeable
species boundaries facilitated by occasional gene-
ow among taxa may be an important
evolutionary mechanism and may well contribute to the adaptive potential of this group
as it allows for the exchange of benecial genetic information between species, e.g. genes
supporting adaptation to new environmental conditions as a result of climate change.
Genetic transfer between lineages may reduce the individual risk of extinction and accelerate
evolutionary rates leading to a higher relative potential for adaptation. Consequently,
evolutionary plasticity facilitated by variation in reproductive traits may raise hope for the
persistence of corals facing future climate change.

Item Type: Thesis (PhD)
Keywords: scleractinian, speciation, phylogeny, life history, taxonomy, Pocillopora, Goniastrea
Copyright Information:

Copyright 2013 the Author

Additional Information:

Chapter 2 is the equivalent of a post-print article published as: Schmidt-Roach, S.; Lundgren, P.; Miller, K. J. ; Gerlach, G.; Noreen, A. M. E.; Andreakis N. (2013), Assessing hidden species diversity in the coral Pocillopora damicornis from Eastern Australia, Coral reefs, 32(1), 161-172. The final publication is available at Springer via http://dx.doi.org/10.1007/s00338-012-0959-z

Chapter 5 is the equivalent of a post-print article published as: Sebastian Schmidt-Roach, S.; Miller, K. J.; Lundgren, P.; Andreakis, N. (2014), With eyes wide open: a revision of species within and closely related to
the pocillopora damicornis species complex (scleractinia; pocilloporidae) using
morphology and genetics. Zoological Journal of the Linnean Society. 170, (1), 1-33

Date Deposited: 26 Feb 2014 00:09
Last Modified: 15 Sep 2017 01:06
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